1. Field of Invention
The present invention relates to hydrodynamic measuring device, and more particularly to a fluidic oscillator for a flowmeter, wherein when a flow of fluid passes through an oscillation chamber of the fluidic oscillator, the flow rate of the fluid is linearly proportional to the frequency of the fluidic oscillation for minimizing the turbulent of the fluid within the fluidic oscillator so as to precisely measure the flow of fluid.
2. Description of Related Arts
A conventional fluidic oscillator is generally employed in a flowmeter for measuring a flow of fluid. The conventional fluidic oscillator generally comprises an oscillator body having two attachment walls 1A defining an oscillating chamber 2A therebetween, an inlet 3A extended from the oscillating chamber 2A, an outlet 4A extended from the oscillating chamber 2A, a splitter 5A provided at the outlet duct 4A, and two feedback channels 6A communicating with the oscillating chamber 2A.
When a flow of fluid passes to the oscillating chamber 2A through the inlet 3A to fill up the oscillating chamber 2A, the fluid is guided to split at the splitter 5A to flow towards the outlet 4A and back to the oscillating chamber 2A through the feedback channels 6A, such that the fluid is started to oscillate within the oscillating chamber 2A.
In order to precisely measure the flow rate of the fluid, the turbulent of the fluid must be minimized. Accordingly, Reynolds number is widely used to determine the turbulent of the fluid, wherein the Reynolds number is implemented as the relation between the dimension of the fluidic oscillator and the velocity of the fluid flow. Therefore, at larger Reynolds numbers, the flow of fluid becomes turbulent.
When the fluid flows into a symmetric divergent or sudden-expansion channel, it often diverts toward either side in a specific range of Reynolds number due to the Coanda effect (Tritton, 1988). Then the flow develops to be either an asymmetric flow structure or a periodically oscillating flow pattern. Previous research reported the specific correlations among the oscillation characteristics and flow parameters (Igarrashi, 2000; Shakouchi, 1989). As the oscillation frequency is linearly proportional to the flow rate, the oscillator could be adopted as a flowmeter. Moreover, these specific correlations are also widely used for atomizers, mixers, and memory and control devices (Groisman et al, 2003).
The conventional fluid oscillator usually provides a measuring range that when the velocity of the flow falls within the measuring range, the conventional fluid oscillator is adapted to precisely measure the flow rate of the fluid. However, if the velocity of the flow is over the measuring range, the flow of fluid becomes turbulent so as to affect the accuracy of the measuring result. As shown in FIGS. 1 and 2, U.S. Pat. 3,902,367 and U.S. Pat. 4,610,162 illustrate the different designs of the dimension of the oscillation chamber of the fluidic oscillator to minimize the turbulent of the fluid.
Based on the operation principles, the fluidic oscillators are categorized as the feedback oscillator, the Karman vortex oscillator, and relaxation oscillator. Although the flow oscillation in a fluidic oscillator is usually initiated by the Coanda effect, the features of oscillation could be significantly altered by the design of feedback channels and the flow control loop.
Tippetts et al. (1973a, 1973b) deduced four major parameters for relaxation type fluidic oscillator, namely, Strouhal number (Str), Reynolds number (Re), Euler number (Eu), and dimensionless control loop inductance (L′), defined as follows.Str=fl/u  (1) Re=ul/v  (2) Eu=2Δp/ρu2  (3) L′=4I′n/(πd′2)  (4) wherein 1 is the characteristic length of the oscillator, f is the frequency of pressure fluctuation, u is the inlet velocity of flow, v is the viscosity, Δp is the pressure loss, and ρ is the fluid density. In addition, I′=I/dn and d′=d/dn. I and d are the length and diameter of the control loop respectively. dn and n are the width and aspect ratio of the inlet port respectively. Tippetts et al. further reported that if the Reynolds number was smaller than a critical value, no more fluctuation occurred. In a certain range of Reynolds number, the Strouhal number remained a constant, and therefore the oscillation frequency was linearly proportional to the flow rate and independent of the fluid properties. In addition, the dimensionless control loop inductance was found to have a linear correlation with the Strouhal number. Wang et al. (1996-1998) combined a vortex amplifier with the oscillator and significantly improved both pressure loss and oscillation spectra of an oscillatory flowmeter. The design was used for remote monitoring of the crude oil pipes. Comparin et al. (1962) pointed out that the oscillator with great depth-to-width ratio inlet nozzle was less affected by the Reynolds number. Yamasaki et al. (1981) systematically evaluated the function of flow splitter while systematically varied its location and length. Honda (2000) reported that when the v-gutter was chosen as the splitters, the Strouhal number became large, but it no longer remained a constant.
The research results on feedback oscillator were relatively less reported (Wright, 1980). However, many related patents have been published in 1975˜2000 (Grant et al., 1975, Adams, 1979; Herzl, 1985; Okabayashi et al., 1986; Stouffer et al., 1998). For the miniature design, Gebhard et al. (1996) used LIGA microfabrication technique to produce a micro oscillator with length 720 sun, width 500 μm, depth 250 μm. Gebhard et al. (1997) further successfully combined the micro oscillator with a micro actuator to be a dynamic micro system. Teser et al. (2000) suggested to replace the Reynolds number with a pressure drop and derived a Teser number for characterizing the micro oscillator.
To extend the application of the fluidic oscillator from the regime of macroscale toward the microscale system need in-depth understandings of hydrodynamics. However, most of the previous work on fluidic oscillator emphasized more on the design rather than the hydrodynamical structure.